Boot into recovery mode (it will require pressing some buttons when rebooting - varies with model but a web search will tell you which), then use the recovery menu to do a factory reset. You lose any data that aren't backed up, but it should clear anything other than damage to the system software (and unless the phone is rooted the only way you could damage that would be a corrupted system update).
You could connect a usb cable and see whether a computer can talk to it if you want to copy stuff off (like photos). No harm trying, though from what you've said I'm not very optimistic.